Well, I still do it for my tweens. They know it isn't real, but they enjoy it. I pretend it's real, they know it is a game. They like to get up in the morning and go looking to seem where the elf is.
Hits in our house have been:
Dad's underpants hung on the Christmas tree (also did toilet paper wrapped around the Christmas tree one year).
Drawing a beard and moustache on family photos (over the glass, obviously, so they can be cleaned off. One year I found Santa hat stickers so put them on the people in photos).
Doing a chocolate drop poo.
Having drawn a moustache on the dc while they were sleeping.
Hanging from the ceiling above the dc's heads when they wake up in a "mission impossible" style (takes multiple days if you have multiple siblings as they all need a turn).
Lazy day options:
Write a joke on the living room mirror in chalk pen.
Wedged in the dc's school shoes so they have to remove them without touching them.
Using a tissue box like a bed.
Piling up loo rolls to make a snow man.
Sitting in the fridge doing nothing in particular.
In sock drawer, using a sock as a sleeping bag.